c++: header-unit build capability [PR 99023]

c++: header-unit build capability [PR 99023]

This defect really required building header-units and include translation
of pieces of the standard library.  This adds smarts to the modules
test harness to do that -- accept .X files as the source file, but
provide '-x c++-system-header $HDR' in the options.  The .X file will
be considered by the driver to be a linker script and ignored (with a
warning).

Using this we can add 2 tests that end up building list_initializer
and iostream, along with a test that iostream's build
include-translates list_initializer's #include.  That discovered a set
of issues with the -flang-info-include-translate=HDR handling, also
fixed and documented here.

	PR c++/99023
	gcc/cp/
	* module.cc (canonicalize_header_name): Use
	cpp_probe_header_unit.
	(maybe_translate_include): Fix note_includes comparison.
	(init_modules): Fix note_includes string termination.
	libcpp/
	* include/cpplib.h (cpp_find_header_unit): Rename to ...
	(cpp_probe_header_unit): ... this.
	* internal.h (_cp_find_header_unit): Declare.
	* files.c (cpp_find_header_unit): Break apart to ..
	(test_header_unit): ... this, and ...
	(_cpp_find_header_unit): ... and, or and ...
	(cpp_probe_header_unit): ... this.
	* macro.c (cpp_get_token_1): Call _cpp_find_header_unit.
	gcc/
	* doc/invoke.texi (flang-info-include-translate): Document header
	lookup behaviour.
	gcc/testsuite/
	* g++.dg/modules/modules.exp: Bail on cross-testing.  Add support
	for .X files.
	* g++.dg/modules/pr99023_a.X: New.
	* g++.dg/modules/pr99023_b.X: New.